Problem: 4 people can paint 5 walls in 32 minutes. How many minutes will it take for 5 people to paint 7 walls? Round to the nearest minute.
Explanation: We know the following about the number of walls $w$ painted by $p$ people in $t$ minutes at a constant rate $r$ $w = r \cdot t \cdot p$ $\begin{align*}w &= 5\text{ walls}\\ p &= 4\text{ people}\\ t &= 32\text{ minutes}\end{align*}$ Substituting known values and solving for $r$ $r = \dfrac{w}{t \cdot p}= \dfrac{5}{32 \cdot 4} = \dfrac{5}{128}\text{ walls painted per minute per person}$ We can now calculate the amount of time to paint 7 walls with 5 people. $t = \dfrac{w}{r \cdot p} = \dfrac{7}{\dfrac{5}{128} \cdot 5} = \dfrac{7}{\dfrac{25}{128}} = \dfrac{896}{25}\text{ minutes}$ $= 35 \dfrac{21}{25}\text{ minutes}$ Round to the nearest minute: $t = 36\text{ minutes}$